The Earth's Atmosphere

The atmosphere surrounds Earth and protects us by blocking out dangerous rays
from the sun. The atmosphere is a mixture of gases that becomes thinner until
it gradually reaches space. It is composed of Nitrogen (78%), Oxygen (21%),
and other gases (1%).

Oxygen is essential to life because it allows us to breathe. In addition,
some of the oxygen has changed over time forming ozone. The the ozone layer
filters out the sun's harmful ultraviolet radiation. Recently, there have been
many studies on how humans caused a hole to develop in the ozone layer.

Humans are also affecting Earth's atmosphere through the greenhouse effect.
Due to increases in gases, such as carbon dioxide, that trap heat being radiated
from the Earth, scientists believe that the atmosphere may have problems balancing
the incoming solar radiation and the reradiated heat from Earth's surface creating
the greenhouse effect .

The atmosphere is divided into five layers depending on how temperature changes
with height. Most of the weather occurs in the first layer.

Layers of the Earth's Atmosphere

The atmosphere is divided into five layers. It is thickest near the surface
and thins out with height until it eventually merges with space.

The troposphere is the first layer above the surface and
contains half of the Earth's atmosphere. Weather occurs in this layer.

Many jet aircraft fly in the stratosphere because it is
very stable. Also, the ozone layer absorbs harmful rays from the Sun.

Meteors burn up in the mesosphere.

The thermosphere is a layer with auroras. It is also where
the space shuttle orbits.

The atmosphere merges into space in the extremely thin exosphere.
This is the upper limit of our atmosphere.

The Troposphere

The troposphere is the lowest region of the Earth's atmosphere, where masses
of air are very well mixed together and the temperature decreases with altitude.

The air is heated from the ground up because the surface of the Earth absorbs
energy and heats up faster than the air. The heat is mixed through the troposphere
because on average the atmosphere in this layer is slightly unstable.

Weather occurs in the Earth's troposphere. In fact, most clouds form in this
layer.

The Stratosphere

In the Earth's stratosphere, the temperature increases with altitude (as opposed
to the troposphere, where it gets colder with altitude).

The increasing temperature in the stratosphere is caused by the presence of
a layer of ozone near an altitude of 25 kilometers. The ozone molecules absorb
high-energy UV rays from the sun, which warm the atmosphere at that level.

The terrestrial stratosphere is the region between the tropopause, or the
end of the troposphere, and the level at which the maximum warming due to the
presence of ozone takes place, which is at an altitude of about 50 kilometers.

Ozone - An Overview

The Ozone Hole. The Ozone Hoax. Pollution. Skin Cancer. The topic of ozone
makes headlines on a regular basis, but why does a single molecule merit such
media coverage? How important is the ozone in our atmosphere and why are scientists
so concerned about its increase near the surface of the Earth and its disappearance
higher up in the atmosphere?

First things first - what is ozone? Ozone is made of three oxygen atoms (O3).
The oxygen in our atmosphere that we breathe is made up of two oxygen atoms
(O2). Because of its chemical formulation, a single atom of oxygen
(O) is unstable. That is, it wants to combine with something else. That is
why oxygen is almost always found in pairs, in its (diatomic) form, where it
is more stable. (O3) is less stable than (O2), because
it wants to return to the diatomic state by giving up an oxygen atom.

When enough ozone molecules are present, it forms a pale blue gas. Ozone has
the same chemical structure whether it is found in the stratosphere or the
troposphere. Where we find ozone in the atmosphere determines whether we consider
it to be Dr. Jekyll or Mr. Hyde.

In the troposphere, the ground-level or "bad" ozone is an air pollutant
that damages human health, vegetation, and many common materials. It is a key
ingredient of urban smog. In the stratosphere, we find the "good" ozone
that protects life on Earth from the harmful effects of the Sun's ultraviolet
rays. We have good reason to be concerned about the thinning of the ozone layer
in the stratosphere. We also have good reason to be concerned about the buildup
of ozone in the troposphere. Although simplistic, the saying "Good up
high and bad near by," sums up ozone in the atmosphere.

Ozone in the Stratosphere

About 90% of the ozone in the Earth's atmosphere lies in the region called
the stratosphere which is found between 16 and 48 kilometers (10 and 30 miles)
above the Earth's surface. Ozone forms a kind of layer in the stratosphere,
where it is more concentrated than anywhere else, but even there it is relatively
scarce. Its concentrations in the ozone layer are typically only 1 to 10 parts
of ozone per 1 million parts of air.

Ozone and oxygen molecules in the stratosphere absorb ultraviolet light from
the Sun, providing a shield that prevents this radiation from passing to the
Earth's surface. While both oxygen and ozone together absorb 95 to 99.9% of
the Sun's ultraviolet radiation, only ozone effectively absorbs the most energetic
ultraviolet light, known as UV-C and UV-B, which causes biological damage.
The protective role of the ozone layer in the upper atmosphere is so vital
that scientists believe life on land probably would not have evolved - and
could not exist today - without it.

The term "shield" as a description of ozone in the stratosphere
is a bit misleading because the molecules do not form an impermeable sphere
around the Earth. Ozone continuously breaks apart into its oxygen atoms and
reforms as ozone molecules, so a particular ozone molecule doesn't last very
long. The "shield" changes constantly, but the atmospheric chemical
processes maintain a dynamic equilibrium that keeps the overall amount of ozone
constant - that is, it would if humans did not contribute to the chemical processes.
There is compelling scientific evidence that ozone is destroyed in the stratosphere
and that some human-released chemicals such as CFC’s are speeding up
the breakdown of ozone in the atmosphere.

While the stratospheric ozone issue is a serious one, in many ways it can
be thought of as an environmental success story. Scientists detected the developing
problem, and collected the evidence that convinced governments around the world
to take regulatory action. Although the global elimination of ozone-depleting
chemicals from the atmosphere will take decades yet, we have made a strong
and positive beginning. For the first time in our species' history, we have
tackled a global environmental issue on a global scale.

The Mesosphere

In the Earth's mesosphere, the air masses are relatively mixed together and
the temperature decreases with altitude. Atmospheric temperatures reach the
lowest average value of around -90°C in the mesosphere. This is also the
layer in which a lot of meteors burn up while entering the Earth's atmosphere.

The mesosphere extends from the top of the stratosphere (the stratopause,
located at about 50 kilometers) to an altitude of about 90 kilometers. Upper
reaches of the atmosphere, such as the mesosphere, can sometimes be detected
by looking at the limb of a planet.

The Thermosphere

The Earth's thermosphere is the layer of the atmosphere which is first exposed
to the Sun's radiation and so is first heated by the Sun. The air is so thin
that a small increase in energy can cause a large increase in temperature.

The temperature in the thermosphere is very dependent on solar activity. When
the Sun is active, temperatures can reach up to 1,500°C or higher!

Because of the thin air in the thermosphere, scientists can't measure the
temperature directly. They measure the density of the air by how much drag
it puts on satellites and then use the density to find the temperature.

The thermosphere also includes the region of the Earth's atmosphere called
the ionosphere. The ionosphere is the region of the atmosphere that is filled
with charged particles. Elevated temperatures can sometimes cause a molecule
to become ionized, therefore, the ionosphere and thermosphere can overlap.

The Ionosphere

Scientists call the ionosphere an extension or a part of the thermosphere.
So technically, the ionosphere is not another atmospheric layer, but a region
of the atmosphere. The ionosphere represents less than 0.1% of the total mass
of the Earth's atmosphere. Even so, it is extremely important!

The upper atmosphere is ionized by solar radiation. Under normal conditions
free electrons and ions tend to recombine and a balance is established between
electron and ion production and loss.

Ionization processes release energy which heat up the upper atmosphere. So
temperature increases with height in the ionosphere region to the extent that
by 150-200km, the Earth's atmosphere is extremely hot compared to surface temperatures.

Different regions of the ionosphere make long distance radio communication
possible by reflecting the radio waves back to Earth. It is also home to auroras
and the mega-ampere currents that heat the atmosphere at high latitudes during
geomagnetically active times. During storms, depletions and enhancements of
ionization occur depending on the local time and geographical location.

Aeronomy is a term of recent origin which is applied to the processes, both
physical and chemical, of the ionosphere.

Regions of the Ionosphere

The ionosphere can be further broken down into the D, E and F regions. The
breakdown is based on what wavelength of solar radiation is absorbed in that
region most frequently or on what level of radiation is needed to photodissociate
the molecules found in these individual regions.

The D region is the lowest in altitude, though it absorbs the most energetic
radiation, hard x-rays. The D region doesn't have a definite starting and stopping
point, but includes the ionization that occurs below about 90km (or ionization
that occurs below the E region).

The E region peaks at about 105km. It absorbs soft x-rays.

The F region starts around 105km and has a maximum around 600km. It is the
highest of all of the regions. Extreme ultra-violet radiation (EUV) is absorbed
there.

On a more practical note, the D and E regions (the lower parts of the ionosphere),
reflect standard AM radio waves back to Earth. Radio waves with shorter lengths
are reflected by the higher F region. Visible light, radar, television and
FM wavelengths are all too short to be reflected by the ionosphere. So these
types of global communication are made possible by satellite transmissions.

The Sun's Effect on the Ionosphere

Invisible layers of ions and electrons are suspended in the Earth's atmosphere
above about 60 kilometers in altitude. The main source of these layers is the
Sun's ultraviolet light which ionizes atoms and molecules in the Earth's upper
atmosphere. During this process, called photoionization, an electron is knocked
free from a neutral atmospheric particle, which then becomes an ion. Because
the Sun's light is responsible for most of the ionization, the ionosphere reaches
maximum densities just after local noon. In this region, at altitudes where
the highest densities occur, about one in every 1000 air particles is ionized.
Resulting ionospheric densities are about a million ions and electrons per
cubic centimeter.

Flares and other energetic events on the Sun produce increased ultraviolet,
x-ray and gamma-ray photons that arrive at the Earth just 8 minutes later and
dramatically increase the density of the ionosphere on the dayside. These solar
events also can produce high velocity protons and electrons (arriving at Earth
hours to days later) that precipitate into the ionosphere in the polar regions
producing large increases in the density of the ionosphere at low altitudes.

The Exosphere

At very high alititudes, the atmosphere becomes very thin. The region where
atoms and molecules escape into space is referred to as the exosphere. This
is the true upper limit of the Earth's atmosphere. The exosphere extends from
the thermosphere out to space.